Currently, liquid crystal has been widely applied in the information display field, and has also made some progress in optical communication applications (S. T. Wu, D. K. Yang. Reflective Liquid Crystal Displays. Wiley, 2001). In recent years, the application field of liquid crystal compounds has been significantly broadened to various types of display devices, electro-optical devices, electronic components, sensors and the like. Nematic liquid crystal compounds have been most widely used in flat panel displays, particularly in TFT active matrix systems.
Liquid crystal display has gone through a long path of development since the discovery of liquid crystals. In 1888, Friedrich Reinitzer, an Austrian botanist, discovered the first liquid crystal material, i.e., cholesteryl benzoate. Manguin invented a rubbing orientation method for producing a single domain liquid crystal and studying optical anisotropy in 1917. E. Bose established Swarm doctrine in 1909, which was supported by experiments of L. S. Ormstein and F. Zernike et al. (1918), and was later explained as statistical fluctuations by De Gennes. In 1933, G W. Oseen and H. Zocher founded continuum theory which was modified by F. C. Frank (1958). M. Born (1916) and K. Lichtennecker (1926) found and studied liquid crystal dielectric anisotropy. In 1932, W. Kast accordingly divided the nematic phase into two categories: positive and negative. In 1927, V. Freedericksz and V. Zolinao discovered that nematic liquid crystal would be deformed and present a voltage threshold (Freederichsz change) in an electric field or magnetic field. This discovery provides a basis for the fabrication of liquid crystal displays.
In 1968, R. Williams in Radio Corporation of America (RCA) found that nematic phase liquid crystals formed stripe domains and had a light scattering phenomenon in an electric field. G. H. Heilmeir then developed this into a dynamic scattering display mode, and made the first liquid crystal display (LCD) in the world. In the early 1970s, Helfrich and Schadt invented twisted-nematic (TN) principle. The combination of the TN photoelectric effect and integrated circuit made a display device (TN-LCD), which has opened up a broad application prospect for liquid crystals. Particularly since the seventies, due to the development of large-scale integrated circuits and liquid crystal materials, the application of liquid crystals has made a breakthrough development in terms of display. Super Twisted Nematic (STN) mode proposed successively by T. Scheffer et al. in 1983-1985 and an Active Matrix (AM) mode proposed by P. Brody in 1972 were re-adopted. Conventional TN-LCD technology has been developed into STN-LCD and TFT-LCD technologies. Although the number of STN scanning lines can reach 768 or greater, there are still problems, such as response speed, viewing angle and gray scale, when the temperature rises. Therefore, for a large area, high information content, color display, an active matrix display mode becomes the first choice. TFT-LCD has been widely used in direct-view televisions, large-screen projection televisions, computer terminal displays and certain military instrument displays. It is believed that TFT-LCD technology will have broader application prospects.
There are two types of “active matrix” including: 1. a metal oxide semiconductor (MOS) on a silicon wafer as a substrate; and 2. a thin film transistor (TFT) on a glass plate as a substrate.
Monocrystalline silicon as a substrate material limits the display size due to the fact that there were many problems occurring at junctions of each part of a display device or even a module assembly. Accordingly, the second type of thin film transistor is a promising active matrix type. The photoelectric effect utilized is generally the TN effect. A TFT includes a compound semiconductor, such as CdSe, or a TFT based on polycrystalline silicon or amorphous silicon.
Currently, the LCD product technologies have been well established and successfully solved technical problems regarding viewing angle, resolution, color saturation, brightness, etc., and the display performance thereof has been close to or superior to that of CRT displays. Large-size and small-to-medium-size LCDs have gradually dominated the flat panel displays in respective fields. However, due to the limitation of (the high viscosity of) the liquid crystal material itself, the response time becomes a principal factor affecting high-performance displays.
In particular, the response time of a liquid crystal is limited by the rotational viscosity γ1 and the elastic constant of the liquid crystal. Therefore, reducing the rotational viscosity of a liquid crystal composition and increasing the elastic constant have a significant effect on reducing the response time of the liquid crystal display and accelerating the response speed of the liquid crystal display.